Have a 1998 Volvo S70 with 128,000 miles. I bought the car less than a year ago and while driving it home picked up a slight vibration in the steering wheel at 70. Changed out he drive axle, driver's side, felt better, but slowly started coming back. Changed out the upper forward motor mount, help some but came back. Slowly started getting worst, dropped highway speed to 60, but started in at 60 and got worst.
The shake usually starts as the car reaches cruise control set speed and last for about a minute, then disappears only to start back after cresting a hill at speed, last about a minute again, then disappears again. The shaking is pretty violent, to the point of the whole car vibrating. Any ideas?

 

engine mount it sounds like to me.

but could be in the ignition system.

I wouldn't rule out tires being out of balance. Tires wear, tread (mass ultimiatly) lost, orgional balance lost. (never seen it, but theory behind it makes sense)

be sure to check all bushings and fasoners on the front suspension and attached to the subframe.

 

The shaking is a side to side shake with a corresponding movement of the steering wheel of about a 1/4" again side to side. It happens just as the power is let off of the front wheels, that point when the car has reached cruise speed and let's off and as the car crests a hill and again the power let's off.
I am going to swap the wheels front to back, shaking in steering should disappear if balance problem. Will check front linkage next......

 

Sounds like it might be alignment related, and when the throttle is reduced, the pulling force from the front wheels is reduced and they are transitioning through an alignment that's causing the wobble. Definitely check your ball joints and linkage, and have an alignment done.

Back when I was into 4x4s and Jeeps, there was a common phenomenon that they called the "Death Wobble", where under certain conditions on lifted Jeeps with bad alignments, at freeway speeds the front wheels would wobble back and forth uncontrollably and make for one heck of a scary ride until you could slow down enough to get it under control. I never experienced it personally, but have heard plenty of stories about it. Sounds like something similar is happening with your car, only not as intense.

Here's an article explaining it a little bit, not all is relevant to our volvos, but some of it is.
http://www.kevinsoffroad.com/how-to-...-death-wobble/

 

New tires and a good balancing solved the problem. Thanks for the input. I swapped the tires front to back and it moved to the rear, Duh. Thanks again guys.